1. Field of the invention
The present invention relates to an electronic control device for an internal combustion engine with an exhaust gas recirculating (EGR) control apparatus, in which a certain controlled variable for controlling the engine, such as an amount of fuel to be supplied to the engine or an ignition timing, is corrected when the EGR control valve is opened.
2. Description of the Related Art
It is widely known to control the engine by controlling an amount of fuel to be supplied into the engine or an ignition timing in response to a detected intake pressure (engine vacuum) and a detected revolutional speed of the engine. The intake pressure is usually detected in units of absolute pressure. Also, it is known to detect the atmospheric pressure and to correct the amount of fuel to be supplied to compensate the air-fuel ratio at an altitude location, as disclosed in Japanese Unexamined Patent Publication No. 58-65950. The feature of this publication is to provide only one pressure sensor in the intake passage of the engine, to make it possible to detect either the intake pressure or the atmospheric pressure by only one pressure sensor. The atmospheric pressure can be detected, for example, by an output from such a pressure sensor, delivered just before start-up of the engine.
An exhaust gas recirculating (EGR) apparatus is also known, which includes a vacuum-operated EGR valve. In the engine including such an EGR apparatus, the amount of fuel also can be controlled in response to the detected intake pressure and the detected revolutional speed. The detected intake pressure is used to detect or represent the amount of the intake air, but the amount of the intake air thus obtained from the detected intake pressure includes a component of the EGR gas (i.e., Q.sub.d =Q.sub.a +Q.sub.e, where Q.sub.d :the detected amount of the intake air, Q.sub.a :the amount of the intake fresh air, Q.sub.e :the amount of the EGR gas). The EGR gas is a burned gas and does not contribute to the combustion, and thus desirably the amount of fuel in proportion to the intake fresh air only is determined. Therefore, it is necessary to correct the amount of the fuel calculated from the detected intake pressure and subtract a component equalling the amount of EGR gas therein.
In this correction step, variations in the amount of the EGR gas caused by changes in the atmospheric pressure upon a change of altitude must be taken into consideration. One of reason for the variations in the EGR is that the flow of the EGR gas is affected by the pressure difference between the exhaust pressure and intake pressure; i.e., it is possible to obtain the same intake pressure at a high altitude as at a normal altitude by opening the throttle valve to a greater extent, but the exhaust pressure will become lower at a high altitude, and thus the flow of the EGR gas will be reduced at a high altitude. Also, the EGR valve is operated by the engine vacuum, and the lift of the EGR valve will vary in accordance with the altitude, for a given constant absolute intake pressure (obtained by opening the throttle valve to a greater extent at a high altitude); i.e., the lift of the EGR valve is less at a high altitude since the atmospheric pressure is applied to the opposite side of the diaphragm of the EGR valve to that subjected to the operating engine vacuum, namely, the EGR valve is also operated by a pressure difference. A typical example of the rate of the EGR gas in the intake air attained at a normal altitude, i.e., under a standard atmospheric pressure, is shown in FIG. 8A. Another example of the rate of the EGR gas in the intake air at a high altitude of 3000 meters, in the same engine, is shown in FIG. 8B. In these FIGS. 8A and 8B, the absolute intake pressure at PM1 is identical.
Japanese Unexamined Patent Publication No. 61-8443 discloses an air-fuel ratio control device for an internal combustion engine with an EGR control valve in which an amount of fuel to be supplied to the engine is basically calculated in response to the detected intake pressure and the detected revolutional speed of the engine and corrected when the EGR control valve is opened. To enable this correction when the EGR control valve is open, the control device in this publication prepares a plurality of two dimensional maps as a function of the detected intake pressure and the detected revolutional speed, for use at various altitudes in response to the detected atmospheric pressure, to thereby maintain the air-fuel ratio at a constant value even when the atmospheric pressure changes at various altitudes. This solution, however, requires a plurality of maps for various atmospheric pressures, and thus a complex control device, and a problem remains in this device in that a precise control cannot be obtained.
Japanese Unexamined Patent Publication No. 61-8444 discloses a similar air-fuel ratio control device for an internal combustion engine with an EGR control valve, in which a correction is carried out in response to a detected difference in the exhaust and intake pressures and the detected revolutional speed. This method does not necessitate a plurality of maps and is simple compared to the former solution, but is still unsatisfactory from the viewpoint of a precise control.
The same problems may occur in the case of the ignition timing.